Light-sensitive silver halide photographic emulsions, used in photography to obtain black and white and color images, consist of light-sensitive silver halide grains dispersed in a hydrophilic dispersing medium. The silver halide grains used in photography typically consist of silver chloride, silver bromide, silver iodide, silver chloro-bromide, silver bromo-iodide, silver chloro-iodide and silver chloro-bromo-iodide. In general, silver bromo-iodide emulsions are more widely used in camera speed photographic elements, such as color photographic elements. Although silver bromo-iodide grains can contain up to 40% iodide moles, which is the solubility limit of silver iodide in silver bromide, lower iodide quantities (e.g. quantities lower than about 20% iodide) are in general used.
Silver halide grains of photographic emulsions have a wide variety of grain shapes. They can have a regular shape, such as cubical or octahedrical, an irregular shape, such as those grains having rounded edges due to ripening effects, or a more or less spherical shape, such as those obtained in the presence of strong ripening agents such as ammonia (see e.g. U.S. Pat. No. 3,894,871 and Zelikman and Levi, Making and Coating Photoqraphic Emulsions, Focal Press, 1964, p. 223).
Tabular silver halide grains, which have two major parallel crystal faces, are known in the art of photography. They have been deeply studied for photographic use e.g. by deCugnac and Chateau, Evolution of the Morphology of Silver Bromide Crystals During Physical Ripening, Science et Industries Photographiques, vol. 33, no. 2, 1962, p. 121-125, by Gutoff, Nucleation and Growth Rates During Precipitation of Slver Halide Photographic Emulsions, Photographic Sciences and Engineering, vol. 14, no. 4, 1970, p. 248-257, in U.S. Pat. Nos. 4,063,951; 4,067,739; 4,150,994; 4,184,877; 4,386,156; 4,434,226; 4,399,215; 4,425,425; 4,400,463; 4,414,306; 4,433,048; 4,478,929 and 4,439,520, in British patent No. 1,150,581 and in German patent applications Ser. Nos. 2,905,655 and 2,921,077.
In the art of light-sensitive silver halide photographic materials, in particular of light-sensitive silver halide photographic materials for color photography, such materials are desired to meet various needs, such as speed, a good image quality (excellent granularity, high definition, low diffusion), a low fog density, excellent latitude in light exposure, sufficiently high optical density, etc. Recently, such needs have become more and more rigid and many improvement techniques have been therefore proposed for use in photographic materials.
In general, in the preparation of light-sensitive silver halide photographic materials, speed and image quality are antagonistic to each other. For example, if speed is increased by increasing the size (volume) of silver halide grains, a decrease in image quality (higher granularity) is often caused. On the other hand, if image quality is to be improved by decreasing the silver halide grain thickness (lower diffusion), thin transparent grains are obtained which have a poor photon-absorption capability and therefore poor sensitivity. The adsorption, on the surface of such grains of sensitizing dye molecules up to a complete coverage of the surface of each single grain by means of a monolayer of such molecules, would make such grains capable of better photon absorption, but the solution of the problem has not yet been described (see The Vogel Centennial, Dye-Sensitization Past and Future, Aug. 26-30, 1973, with particular reference to G. R. Bird's final reading). Consequently, it is highly desirable to provide silver halide photographic elements which allow to obtain both improved sensitivity and image quality.